There exists a conventionally known technique for achieving an improvement in fuel economy by performing combustion in a state of “lean” air-fuel ratio in which air-fuel mixture in individual cylinders of a spark-ignition engine is burnt at an air-fuel ratio larger than the stoichiometric air-fuel ratio. One example of this kind is shown in Japanese Unexamined Patent Publication No. H10-274085, which employs fuel injectors for injecting fuel directly into combustion chambers to cause stratified charge combustion by injecting fuel during a compression stroke in a low-speed, low-load range, for example, to thereby accomplish extremely lean mixture combustion.
In this kind of engine, it is impossible to achieve sufficient emission-cleaning performance with respect to nitrogen oxides (NOx) under lean burn operating conditions by using an ordinary three-way catalyst alone, which is a catalyst having high performance to convert hydrocarbons (HC), carbon monoxide (CO) and NOx at about the stoichiometric air-fuel ratio, as an emission-cleaning converter. Therefore, as shown in the aforementioned Publication, the engine is provided with a lean NOx catalyst which adsorbs NOx in an oxygen-rich atmosphere and releases and reduces NOx in an atmosphere where oxygen concentration has decreased. If the amount of NOx adsorbed by the lean NOx catalyst has increased under the lean burn operating conditions when the lean NOx catalyst of this kind is being used, the fuel is injected not only for primary combustion but an additional amount of fuel is injected during an expansion stroke to decrease the air-fuel ratio and generate CO for accelerating release and reduction of NOx as shown in the aforementioned Publication, for example.
The aforementioned engine which performs conventional lean burn operation requires the lean NOx catalyst to provide NOx-converting performance during the lean burn operating conditions. This type of engine also requires the three-way catalyst for cleaning emissions in such engine operating ranges as a high-load range in which the engine is operated at the stoichiometric air-fuel ratio. The lean NOx catalyst provided along with the three-way catalyst needs to have a relatively large capacity to provide a capability to adsorb a certain amount of NOx and is expensive as compared to the three-way catalyst, so that the provision of this lean NOx catalyst is disadvantageous from the viewpoint of product cost.
In addition, it is necessary to temporarily decrease the air-fuel ratio by feeding additional amounts of fuel to accelerate release and reduction of NOx at specific intervals of time when the amount of NOx adsorbed increases as stated above in order to maintain the converting performance of the lean NOx catalyst. This would jeopardize fuel economy improvement effect offered by lean burn operation.
Furthermore, the lean NOx catalyst is susceptible to poisoning by sulfurization when used fuel contains high sulfur content. The lean NOx catalyst should therefore be subjected to regeneration treatment, such as catalyst heating and feeding of a reducing agent, to prevent this sulfur-poisoning problem. This regeneration treatment of the lean NOx catalyst is likely to cause a reduction in the fuel economy improvement effect and deterioration of its durability.
The invention has been made in consideration of the aforementioned problems of the prior art. Accordingly, it is an object of the invention to provide a control device of a spark-ignition engine capable of providing improved emission-cleaning performance by use of a three-way catalyst alone, without the need for a lean NOx catalyst, while ensuring a fuel economy improvement effect of lean burn operation.